1. Field of the Invention
The present invention relates to a lithium salt, an ionic conductor and a liquid electrolyte which can be used for polymer solid electrolytes and liquid electrolytes of lithium secondary batteries.
2. Description of the Related Art
Since lithium salts, which have been used for electrolytes of lithium secondary batteries conventionally, are solids which have a chemical structure of high dissociation, they do not exhibit ionic conduction independently. Hence, in order to give them ionic conduction, they have been conventionally dissolved in appropriate solvents to use.
Incidentally, from the viewpoint of inhibiting reactions with electrode active materials, nonaqueous solvents have been used in general for solvents in which lithium salts are dissolved. Ordinary nonaqueous solvents are combustible. Accordingly, in lithium secondary batteries which are expected to be applied to consumer appliances, nothing can be better than to avoid using such nonaqueous solvents if possible at all.
Hence, in order to keep nonaqueous solvents from using, it has been carried out forming solid electrolytes by dissolving lithium salts in appropriate polymers. Currently, as solid electrolytes having been reported so far, systems have been investigated in which alkali metals of good dissociation are dissolved in ether-based polymers, being cationic conductors. Since solid electrolytes are of high safety, in addition, since they are good in terms of formability to film, lightweightness, flexibility, and elasticity, they are expected to be more promising in the future.
However, although sold electrolytes, in which lithium salts are dissolved in appropriate polymers, exhibit high ionic conductivities, they exhibit lithium ion transport number of 0.5 or less. In particular, they exhibit much lower lithium ion transport number in polyether. Moreover, when conventional lithium salts are dissolved in solid electrolytes, not only cations move well, but also anions move well. Accordingly, the deposition of cations onto cathode takes place so that there arises a problem in that the ionic conductivity lowers. It is believed that the problem could be solved by restricting the movement of cations, specifically by employing single ionic conductors, in which cations move preferentially, as the lithium salt. However, since single ionic conductors fasten paired anions, the conduction of cations has been restricted as well so that the ionic conductivities lower.
Moreover, molten lithium salts have been known which turn into liquid at room temperature independently. However, the conducting species is anions, not cations. In addition, the present inventors reported a polymer electrolyte which has an aluminate structure. However, not only the method of synthesizing the polymer electrolyte has been complicated, but also the ionic conductivity has not been sufficiently high.